Brake fluid resevoir lid

I was trying to check my brake fluid today, but the metal cap just came off in my hand leaving behind a rubber lid of some sort. When I tried to open it the rubber split.
What am I doing wrong? and anyone have a good source for a new rubber piece?
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Yeah, I found reservoirs on superperformance and mrfiat. But they both looked different than what I have. Seem to have some sort of electrical connections mine doesn't have.
I only need the cap and I think I found one that will work. Seems to be the same reservoir as a Alfa https://mrfiat.com/products/alfa-romeo-giulia-sedan-double-circuit-brake-fluid-reservoir-cap